The present invention is generally directed to the recording and reproduction of signals on a magnetic medium, particularly the positioning of a record/reproduce transducer head adjacent to a track of information on a magnetic recording tape. More specifically, the invention is directed to a system for automatically positioning plural transducing heads adjacent to tracks of video signal information that is recorded in a segmented format.
Information signals, for example video signals, are typically recorded on a magnetic medium, such as magnetic tape, in discrete tracks of information. In one type of recording system that is in widespread use for recording video signals, the magnetic tape is disposed around the periphery of a scanning drum and longitudinally transported relative thereto. One or more magnetic transducing heads rotate about the circumference of the drum. The tape follows a helical path around the drum, so that the rotating head transcribes a path, or track, along the tape that is disposed at an angle relative to the longitudinal direction of the tape. As the tape is transported around the drum at a predetermined speed, successive adjacent tracks are formed on the tape at that angle. During playback, if the tape is transported around the scanning drum at the same speed, the rotating transducing head will successively read the tracks in the order in which they were recorded.
To provide special effects, such as slow motion, stop frame, reverse motion, etc., the tape is transported past the scanning head at a speed different from the normal record and play speed. Due to the change in relative speed between the tape and the rotating head, the path circumscribed by the head does not precisely follow the track recorded on the tape. More precisely, the angle at which the head traverses the tape will be different from the angle of the recorded track. The magnitude of this angular difference will be dependent upon the deviation of the tape speed from the normal record and playback speed.
Accordingly, in order to faithfully reproduce individual tracks of video information, it is necessary to deflect the transducing head in a direction that is substantially transverse to its path of movement around the scanning drum. In other words, the transducing head must be moved in a direction that is parallel to the axis of the drum to enable it to remain adjacent to a particular track of recorded information. The position of the head in this direction is sometimes referred to as its "elevation." Exemplary servo systems for controlling the elevation of the transducing head are disclosed in U.S. Pat. Nos. 4,151,570, 4,163,993 and 4,485,414, among others.
Generally speaking, head elevation servo systems have two basic modes of operation. One mode, which might be viewed as a "coarse" control mode, involves the general positioning of the head at a location where the recorded track is expected to be. More particularly, the longitudinal speed of the tape, relative to normal play speed, provides an indication of the deviation between the angle of a track recorded on the tape and the normal path followed by the transducing head. Thus, based upon the ratio of actual tape speed to normal play speed, or other factors related to this ratio, the location of the track at any point in the rotational movement of the transducing head can be predicted, and the head positioned at this predicted location. Typically, the elevation of the head is controlled by means of a voltage signal. In operation, a ramp voltage can be generated whose slope is proportional to the ratio of actual tape speed to normal tape speed and whose direction is dependent upon whether the actual tape speed is slower or faster than normal play speed. By applying this ramp voltage to the head position servo system, the head can be positioned at the predicted location for the track. Thus, if the tape is being transported at normal play speed, the ramp would have a slope of zero, so that the transducing head is at a center elevation where it traverses its normal path around the scanning drum.
Within this coarse control mode of operation, a fine positioning of the transducing head is also carried out. More particularly, due to varied conditions such as stretching of the tape, differences in the normal speed between one machine and another, etc., the transducing head may not be precisely positioned over a recorded track. As the location of the head moves away from the center of the track, the quality of the reproduced signal begins to degrade. Thus, various techniques have been developed to maintain the transducing head substantially centered over the recorded track. One popular technique imposes a continuously oscillating dither signal on the ramp voltage to provide feedback information that enables the transducing head to be precisely positioned. Further disclosure regarding the use of the dither signal is provided, for example, in the aforementioned U.S. Pat. No. 4,151,570.
The present invention is particularly concerned with the first mentioned mode of operation of a head position servo system, i.e. the general control of head elevation based upon a predicted location of the recorded track. Therefore, further discussions will be focussed upon this aspect of the servo system.
As noted above, the general positioning of the head is carried out by generating a ramp voltage whose slope is related to the ratio of actual tape speed to normal play speed. In addition, the voltage must be periodically reset between tracks to assure proper positioning. For example, in the stop, or freeze frame mode, the same field or frame of video information must be continuously reproduced. In one class of recording, each field is recorded on a single track. Therefore, the same track must be continuously reproduced to display a single field, or two adjacent tracks must be alternately reproduced to display a complete frame, for a normal television signal which comprises a sequence of frames, each consisting of an odd field and an even field, according to the television standard RS170A. During each reproduction of a track, i.e., display of a field, the transducing head is displaced by the head position servo system due to the fact that the tape is not being transported at its normal play speed. Therefore, at the end of each track the head must be repositioned for the beginning of the next track. This repositioning is carried out by resetting the ramp voltage to the same value at the beginning of each track.
A similar type of operation is carried out in other viewing modes. For example, if the tape is being played at 2/3 normal speed, every other field is repeated once. Thus, the head would be positioned to reproduce two fields in succession, and then reset to reproduce the second field. Similarly, if the tape is being played at twice the normal speed, every other field must be skipped. Therefore, after each track is reproduced, the head must be reset to jump the next adjacent track and begin reproduction of the following track.
In the past, video signals have typically been recorded on the tape in an analog format. In this format, each field is recorded on one track of information, which is reproduced during one revolution of the scanning drum. At the end of each revolution, the appropriate action can be carried out to properly position the head prior to the scanning of the next track of information.
More recently, however, video signals have begun to be recorded on tape in a digital format. In the format known as the D-2 format, there is not a one-to-one correspondence between a track of information and a video field. Rather, the digital information pertaining to one video field is typically divided into three or four segments, and each segment is recorded on a separate track. Thus, the display of a single video frame or field involves the reproduction of plural tracks on the tape. In this type of operation, it is not possible to reset the position of the head between any two tracks. Rather, the resetting of the head must be coordinated with the segmentation of the field to ensure proper display of the field. For example, if a field of video information is divided into three segments, the head should not be reset between the segments pertaining to a single field. If it were, the top segment of the video display may relate to one field whereas the bottom segment could pertain to a different field, resulting in a distorted picture. Thus, the resetting of the head must be coordinated with the segmentation of the field.
A further consideration that requires attention in digital recording is the fact that the segmented video information is recorded and reproduced with more than one head. For example, the scanning drum may have two heads which are located 180.degree. apart from one another. Typically, the magnetic tape is only in contact with one half of the scanning drum in this type of arrangement. Thus, one head will be in contact with the tape while the other head is not. As one head concludes the recording or reproduction of a track and begins to come out of contact with the tape, the other head comes into contact with the tape and begins the recording or reproduction of the next successive track.
This type of arrangement presents additional difficulties with regard to the proper positioning of the heads. More particularly, when a head is in contact with the tape, the quality of the video signal that is reproduced can be used as an indicator of the location of the head relative to the track. However, when the head is out of contact with the tape, this type of feedback is not available.
In the past, this difficulty was addressed by returning the head to a predetermined reference position each time it came out of contact with the tape. The head remained in this reference position during the time that it was out of contact, and then it was quickly shifted to an appropriate position to begin scanning the next track when it was brought back into contact with the tape. It can be appreciated that this type of operation requires two resets for each track that is to be scanned, i.e., one reset at the end of the track to bring the head to the reference position and a second reset at the beginning of the next track to bring the head from the reference position to the appropriate position for initial scanning of that track.
The systems that are in present use to position the transducer head are mechanical in nature. Typically, they might include a piezoelectric element whose physical shape is changed in response to an electrical control signal, or a voice coil which is deflected by the control signal. Being mechanical in nature, they have inherent inertia which limits the speed at which the head can be repositioned from one location to another. In addition, they may become subject to wear and/or drift after repeated use. A system which requires frequent and rapid resetting of the head position, such as that referred to above, is limited by the physical constraints of mechanical head positioning systems.
Accordingly, it is desirable to provide a head position servo system which is suitable for use with digital video signals that are recorded in a segmented format. Further along these lines, it is desirable to carry out accurate elevational positioning of a magnetic transducing head in a head position servo system with a minimum number of resets being required for any particular playback mode, to thereby reduce the wear on the head positioning system.